Binding affinities of insulin-like growth factor-I (IGF-I) fusion proteins to IGF binding protein 1 and IGF-I receptor are not correlated with mitogenic activity

In this report, comparisons between molecular affinities and cellular proliferation activities have been made for insulin-like growth factor-I (IGP-I) and two IGF-I fusion proteins in order to evaluate fusion proteins as tools for receptor binding studies, Binding affinities and growth promoting effects of the N-terminal fusion Z-IGF-I and the C-terminal fusion IGF-I-Z, and native recombinant human IGF-I, were analyzed, Binding kinetic properties of the three IGF-I variants mere analyzed using BIAcore kinetic interaction analysis testing for binding to both human IGF binding protein 1 (IGFBP-1) and a soluble form of the human IGF type I receptor extracellular domains (sIGF-I-R), The growth promoting effects on SaOS-2 human osteosarcoma cells of the different fusion proteins were analyzed, A comparison of receptor binding affinities and growth promoting effects shows that the fusion protein receptor affinity does not correlate with proliferative potential, The IGF-I-Z fusion, with the lowest receptor affinity, shows similar proliferative potential to native IGF-I, However, the Z-IGF-I fusion protein, with twice the receptor affinity of IGF-I-Z, displays only about 70% of the IGF-I-Z growth promoting activity, Both IGF-I fusion proteins possess similar affinity to IGFBP-1, These results indicate that determinants other than the receptor affinity could be involved in the regulation of IGF-I proliferative action, This study demonstrates that ligand fusion proteins may be useful to study mechanisms of ligand induced receptor activation. (C) 1997 Federation of European Biochemical Societies.